1. Field of the Invention
The present invention relates to an electrochemical cell and a fuel cell using the same. More specifically, the present invention relates to an electrochemical cell which is applicable to an energy device such as a fuel cell, a lithium ion cell, or a dye-sensitized solar cell and a fuel cell using the same.
2. Description of the Related Art
In recent years, solving environmental and energy problems is a significant challenge in countries consuming a lot of energy.
Fuel cells, which have high power generation efficiency and are excellent in reducing environmental burden, are next-generation energy supply devices expected to contribute to the solution of these problems. Moreover, the fuel cells are classified by the types of electrolyte, among which polymer electrolyte fuel cells are small and can provide high power. Accordingly, studies and developments are being conducted in applications of the polymer electrolyte fuel cells to small-scale stationary energy sources and energy sources for movable bodies and mobile phones.
Such solid polymer electrolyte is a material including a hydrophilic functional group such as a sulfonic acid group or a phosphoric acid group in a polymer chain. The solid polymer electrolyte is tightly bound to particular ions and has a property of selectively transmitting anions or cations. The solid polymer electrolyte is therefore shaped into particles, fibers, or membranes and utilized for various applications such as electrodialysis, diffusion dialysis and battery diaphragms.
The polymer electrolyte fuel cells are being increasingly improved at present as power generation means which can provide a high total energy efficiency. The main constituent elements thereof are both electrodes of an anode and a cathode, a separator plate forming a gas passage and a solid polymer electrolyte membrane which separates the both electrodes. Protons produced on a catalyst of the anode move inside the solid polymer electrolyte membrane and reach a catalyst of the cathode to react with oxygen. Resistance to ionic conduction between the both electrodes therefore significantly affects cell performances.
To form a fuel cell using the aforementioned solid polymer electrolyte membrane, it is necessary to connect the catalysts of the both electrodes and the solid polymer electrolyte membrane by means of an ion conduction path. The electrodes therefore include catalyst layers formed by mixing a polymer electrolyte solution and catalyst particles for applying and drying to bind the same. Such electrodes and a solid polymer electrolyte membrane are then heated and pressed, thus obtaining the fuel cell.
The polymer electrolyte which plays a role in ionic conduction generally includes a polymer having a sulfonic acid group introduced into a perfluorocarbon main chain. Specific commercial products thereof are Nafion (DuPont Kabushiki Kaisha), Flemion (Asahi Glass Co., Ltd.), Aciplex (Asahi Kasei Corporation), and the like.
The perfluorosulfonic acid polymer electrolyte is composed of main chains of perfluorocarbon and side chains each including a sulfonic acid group. The polymer electrolyte is considered to be microphase-separated into a region mainly composed of the sulfonic acid groups and a region mainly composed of the perfluorocarbon main chains, and the phase of the sulfonic acid groups forms a cluster. The part of the perfluorocarbon main chains aggregated contributes to chemical stability of the perfluoroslufonic acid electrolyte membrane. The part in which the sulfonic acid groups aggregate to form a cluster contributes to the ionic conduction.
However, such a perfluorosulfonic acid electrolyte membrane having both excellent chemical stability and ionic conductivity is difficult to manufacture and has a disadvantage of being very expensive. The perfluorosulfonic acid electrolyte membrane has therefore limited use and is very difficult to apply to polymer electrolyte fuel cells which are expected as power sources for movable bodies.
Moreover, current polymer electrolyte fuel cells are operated in a comparatively low temperature range of about 80° C. This is because the used fluorine membrane, which has a glass transition point around 120 to 130° C., cannot maintain the ion channel structure contributing to proton conduction in a range of temperature higher than the glass transition point and is substantially desired to be used at a temperature of not higher than 100° C. As another reason, since water is used as a proton conducting medium, pressurization is required when the temperature exceeds 100° C., which is a boiling point of water, thus increasing the apparatus size.
When the operation temperature is low, the power generation efficiency of the fuel cell is low, and the catalyst is notably poisoned by carbon monoxide. When the operation temperature is 100° C. or more, the power generation efficiency is increased, and furthermore, waste heat can be utilized, enabling efficient use of energy. Considering applications to fuel cell vehicles, if the operation temperature can be raised to 120° C., the power generation efficiency can be increased, and moreover, radiator load necessary to exhaust heat can be reduced. It is therefore possible to apply a radiator of the same specifications as those of the radiators used in current mobile bodies, thus reducing the system size.
As described above, to realize operation at higher temperature, various examinations have been made. Typically, as an action with an eye to reducing cost of the aforementioned electrolyte, considerations are being made for applications of aromatic hydrocarbon polymer materials, which are cheap and excellent in heat resistance, to the solid polymer electrolyte instead of the fluorine membranes.
For example, as the material of the solid polymer electrolyte, various types of aromatic hydrocarbon are being examined, such as sulfonated polyether ether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and polybenzimidazol (see the Japanese Patent Unexamined Publication Nos. H6-93114, H9-245818, H11-116679, H11-67224, and H9-73908 and the Japanese Patent Translation Publication No. H11-510198). However, water is used as the proton conducting medium also in the aforementioned polymer electrolytes. Accordingly, when the operation temperature exceeds 100° C., which is the boiling point of water, pressurization is required, and the apparatus size is increased.
Furthermore, it was proposed to apply an electrolyte including an ionic liquid to a fuel cell as an electrolyte allowing proton conduction under conditions of high temperature and no humidification (see the Japanese Patent Unexamined Publication No. 2003-123791). Using the ionic liquid for the electrolyte of the fuel cell can provide high proton conductance without depending on water.
Still furthermore, it was proposed to apply to a fuel cell an electrolyte including a protic ionic liquid composed of alkylamine and oxo-acid both have same alkyl groups (see Nakamoto Hirohumi, Matsuoka Hideyuki, and Watanabe Masayoshi, “Properties of protic ionic liquids consisting of alkyl amines and oxo-acids”, Abstracts of the 73th Meeting of the Electrochemical Society of Japan, 2006, P147).